1. Field of the Invention
The present invention relates to paperforming fabrics as used in the wet end of papermaking machines. In the wet end paper pulp, or stock, is evenly distributed over a foraminous belt-like fabric, and water in the stock drains through the fabric to set up, or form, the initial paper web. Fourdrinier and Vertiformer machines are two typical types of wet end machines, and paperforming fabrics for Fourdriniers may be as wide as 400 inches and as long as two hundred feet. These belt-like fabrics travel over rolls, foils, suction boxes and other machine elements up to speeds of 4,000 feet per minute, and when propelled at such velocities the stresses developed in the fabric threads become very substantial.
Forming fabrics are woven on a loom from fine threads numbering from about forty to one hundred twenty threads per inch. The open area of a single layer fabric is typically 18 to 24 percent of the total fabric area, and the thread structure and minute openings between threads must be uniform across the entire fabric to obtain proper drainage and paper formation. They are, therefore, a precisely manufactured product that must exhibit several characteristics to achieve satisfactory operation.
Paperforming fabrics, for example, must not stretch excessively under the applied tension loads required to drive them along and through a paperforming machine, for otherwise they may lose tight engagement with the driving rolls and slip. They must not contract excessively in width, nor can they wrinkle across their surfaces as tensions change along their route in the papermaking machine. The knuckle formation of the threads must be uniform and there cannot be any blemishes in the fabric surface that might mark paper being produced. The water drainage rate through a fabric must also be carefully controlled, and each papermaking machine has its own, individual characteristics that must be satisfied, so that fabrics are usually designed for a specific machine.
The paper supporting surface of a fabric must allow the paper sheet to lift freely off the fabric as it passes to the next stage of the papermaking process. Also, the fabric must be able to withstand abrasive wear resulting from friction with the machine elements, so as to have a satisfactory life. As wear occurs, the threads in the machine direction, i.e. the direction of travel, must retain sufficient cross section area to withstand the tension loading to which they are subjected in pulling the fabric through the machine. Consequently, it is commonly necessary to relieve the machine direction threads from supporting the fabric on the machine.
To meet the foregoing requirements, a thread material must be selected that does not have excessive elasticity or plasticity, and the machine direction and cross direction threads must have interlocking crimp that bind them in place, so that they do not shift relative to each other, and so that they do not straighten out or undergo a crimp interchange in response to tension loads. A stable fabric can then be provided that does not excessively stretch or narrow, and in which the individual threads retain their relative positions.
The knuckle heights of the machine direction and cross direction thread systems also must be carefully controlled. For example, on the wear, or bearing side of the fabric predominant knuckles are commonly created in the cross direction threads to absorb a greater proportion of the wear, thereby protecting the tension loaded machine direction threads. On the paperforming side, the knuckle crests of the two thread systems are commonly brought into a common plane to provide a smooth paper supporting surface relatively free from excessive marking of the paper sheet. In some applications knuckle configurations are varied to achieve other specific goals. For example, long cross machine direction knuckles on the paper side may be raised to improve crosswise alignment of the paper fibers, or other knuckle arrangements may be employed to effect better sheet release.
It is seen that the field of manufacture of paperforming fabrics concerns a precise product having stringent requirements for satisfactory production of paper.
2. Description of the Prior Art
Traditional paperforming fabrics were flat woven on specially constructed looms from metal threads. Bronzes were typically used for the warp threads, which became the machine direction threads on the papermaking machine, and brass was used for the weft threads, which were the cross machine direction threads on the papermaking machine. Other metallic materials were used for special fabric constructions. In earlier times, these metal fabrics were traditionally woven in a plain weave, with the threads of each of the warp and weft thread systems interlacing through the fabric in a one over-one under pattern. A plain weave utilizes two harnesses in the loom for changing the warp shed, and can therefore be termed a two harness weave.
In time, the semi-twill weave became more popular, using three harnesses in the loom for changing the warp shed. In the semi-twill, the threads of each thread system interlace through the fabric so as to pass to one side of a thread of the opposite thread system and then to the other side of a pair of threads of the opposite thread system. Occasionally, metal fabrics also embodied a four-harness weave in a two over-two under twill pattern. However, because of the inherent dimensional stability exhibited by metal threads there was little experimentation or use beyond the semi-twill weave.
In the late 1950's and ensuing 1960's synthetic thread materials came into use, and have now largely supplanted metal. Polyester and polyamide materials in monofilament and multifilament threads have been preferred, with polyester monofilament threads being the most widely accepted forming fabric material today. Synthetics have quite different elastic and plastic properties than metal, and manufacturing a synthetic fabric requires an additional step of heat setting under tension after weaving to dimensionally stabilize the fabric. By controlling the processes of weaving and heat setting, and by selecting thread diameters and mesh counts, persons skilled in the art have come to exercise significant control over fabric characteristics such as the fabric modulus, thread crimp, and knuckle formation.
With the advent of synthetic materials, new weave patterns have come into predominant use. Four-harness weaves are commonly employed, in which individual threads in both thread systems pass first to one side of a thread of the other thread system, and then interlace through the fabric to pass to the opposite side of three threads of the other thread system. Thus each thread forms knuckles on one face of the fabric comprised of a single crossover, and long knuckles on the opposite fabric face of three crossovers. Some of the four-harness weaves are in a regular twill pattern, in which the knuckles comprising single thread crossovers are in a one-two-three-four sequence. Wider use has been made of the broken satin pattern, in which the knuckles comprising a single thread crossover are in a one-three-two-four sequence. Also, some two over-two under four-harness fabrics have been utilized in both regular twill and broken satin.
Synthetic fabrics have also been constructed in five-harness weaves in a satin pattern. Five-harness satin patterns have their knuckles of a single thread crossover woven in a one-three-five-two-four sequence, or a one-four-two-five-three sequence. Weaves employing greater than five harnesses have also been suggested, and the term "Atlas" has been used to generally denote fabrics of five or a greater number of harnesses in which each thread of each thread system passes to one side of a single thread of the other thread system and then interlaces through the fabric to pass in a long knuckle, or float to the other side of the remaining threads of the other thread system in the weave repeat. Some usage has also been made of five-harness fabrics constructed in two over-three under patterns which produce a regular twill pattern.
As used herein, the term "regular twill" means a fabric pattern having a succession of adjacent threads that present on a fabric face equal length knuckles comprised of two or more crossover in which each successive thread advances its weave repeat by one crossover from the preceding thread, to form the characteristic diagonal line that distinguishes a twill. In many instances a regular twill pattern has the disadvantage of producing a corresponding mark on the paper surface which can be objectionable. Other patterns resembling a regular twill may also at times produce similar paper marking. For example, a four-harness satin weave may produce a paper mark somewhat characteristic of a regular twill weave marking. As discussed hereinafter, it is an objective of the present invention to reduce such marking, while at the same time preserving other desirable characteristics for a paperforming fabric.